Ionization CI

In chemical ionization-mass spectrometry (CI-MS), the sample molecules are combined with a stream of ionized reagent gas that is present in great excess relative to the sample. When the sample molecules collide with the preionized reagent gas, some of the sample molecules are ionized by various mechanisms, including proton transfer, electron nansfer, and adduct formation. Almost any readily available gas or highly volatile liquid can be used as a reagent gas for CI-MS. 

Common ionizing reagents for CI-MS include methane, ammonia, isobutane, and methanol. When methane is used as the Cl reagent gas, the predominant ionization event is proton transfer from a CHs ion to the sample. Minor ions are formed by adduct formation between C2H5+ and higher homologues with the sample. The methane is converted to ions as shown in Equations 3.1-3.4. 

The sample molecule M is then ionized through the ion-molecule reactions in Equations 3.5 and 3.6  

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The situation is very similar for Cl with ammonia as reagent gas (Equations 3.V-3.9)  

Using isobutane as reagent gas produces fert-butyl cations (Equations 8.10 and 8.11), which readily protonate basic sites on the sample molecule (Equation 8.12). Adduct formation is also possible using isobutane in CI-MS (Equation 8.13). 

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An abundant molecular ion may indicate that an aromatic compound or highly unsaturated ring compound is present. If no molecular ion is observed and one cannot be deduced, the use of chemical ionization (ci), negative chemical ionization (nci), fast atom bombardment (FAB), or electrospray ionization (ESI) should provide a molecular ion. 

An EPA method was created for measuring NDMA and six additional nitrosamines in drinking water (EPA Method 521) [55]. This method uses GC/ chemical ionization (CI)-MS/MS and enables the measurement of NDMA and six other nitrosamines (N-nitrosomethylethylamine, N-nitrosodiethylamine, N-nitroso-di-n-propylamine, N-nitroso-di- -butylamine, N-nitrosopyrrolidine, and N-nitrosopiperidine) in drinking water at detection limits ranging fi om 1.2 to 2.1 ng/L. A liquid chromatography (LC)/MS/MS method [56] can also be used to measure nine nitrosamines, including N-nitrosodiphenylamine, which is thermally unstable and cannot be measured using the EPA Method. 

Figure 2 Contributions to the total stopping cross section 5t from ionization Ci by and H°, electron capture cec by H, electron loss cel by H°, and excitation o-gx by protons in water. The data are from a compilation by Uehara et al. [14], The ICRU recommendation 5t-ICRU is from Ref 5.

Cl = chemical ionization CI-NI = monitoring negative ions in the chemical ionization mode GC = gas chromatogmphy HPLC = high performance hquid chromatography MS = mass spectrometry pmol = picomole SPE = solid-phase extraction TSD = thermionic specific detection... 

Chemical ionization (CI) uses a reagent ion to react with the analyte molecules to form ions by either a proton or hydride transfer. Reagent ions are produced by introducing a large excess of gas, such as methane, into an El ion source. Electron collisions produce CH/ and CH,, which further react with methane to form CHs and C2H5. ... 

Ion genera lion can be achieved in a number of ways electron impact (Eh ionization, chemical ionization (CI). fas I atom bombardment (FAB), matrix assisted taser desorption ionization (MAI.DI), eleclrospray ionization (ESI) and atmospheric pressure chemical ionization (APC I are the most common methods,... 

Chemical ionization (CI)-MS can be used to study alkaloids that are not amenable to examination by electron impact (EI)-MS. For example, the quaternary alkaloid thalirabine (Section II,C, 123), undergoes fragmentation under the conditions of EI-MS and does not show a parent ion however, the CI-MS shows a double Hofmann elimination product which retains the skeletal atoms (32). Field desorption (FD)-MS has similar utility, as in the case of cycleanine IV-oxide (Section II,C,17) for which FD-MS shows the parent ion not detectable by EI-MS (65). Desorption/CIMS (D/CIMS) was used on dihydrosecocephar-anthine (Sec. H,C,30) and related bases (80,292a). 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (CI), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

Biological specimen extraction can be accomplished by liquid-liquid, solid-phase or solid-phase microextraction with subsequent detection of GHB or GBL by gas chromatography-mass spectrometry (GC-MS) using electron ionization (El), positive or negative chemical ionization (CI) or gas chromatography with flame ionization detection (GC-FID). LeBeau et al. (1999) describes a method that employs two ahquots of specimen. The first is converted to GBL with concentrated sulfuric acid while the second is extracted without conversion. A simple liquid-liquid methylene chloride extraction was utilized, and the ahquots were then screened by GC-FID without derivatization. Specimens that screened positive by this method were then re-aliquoted and subjected to the same extraction with the addition of the deuterated analog of GBL. The extract was then analyzed by headspace GC-MS in the full-scan mode. Quantitation was performed by comparison of the area of the... 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

Ionization methods that are based on the first approach are electron ionization resonance electron capture (REC), chemical ionization (CI), negative ion chemical ioniza-... 

To perform MS, one must make ions from neutral molecules. Ionization methods have advanced from the classic electron ionization (El), through chemical ionization (CI), field desorption (FD), fast atom bombardment (FAB) and thermospray to the atmospheric pressure ionization (API) techniques currently favored. El is classic, but is restricted in its applicability to thermally stable, volatilizable compounds. ED was always a specialized niche technique applicable to some larger compounds. EAB enjoyed a meteoric rise and fall in use first reported in 1981, but has all but disappeared now, being replaced by the API techniques atmospheric pressure Cl (APCI) °° and electrospray ionization (ESI). ° Matrix-assisted laser desorption ionization (MALDI) has shown significant utility for characterizing larger proteins, approximately 100 kDa and larger. 

Chemical ionization. CI-MS complements El by giving the molecular weight of the compound. Direct Cl of underivatized dodecyl maltoside with ammonia showed strong quasimolecular ions and weak fragments corresponding to scission of the molecule into its sugar and alkyl constituents (97). 

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